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Message-ID: <20181022094240.mp7pd2m2abyttrfb@brauner.io>
Date: Mon, 22 Oct 2018 11:42:41 +0200
From: Christian Brauner <christian@...uner.io>
To: Tycho Andersen <tycho@...ho.ws>
Cc: Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>,
Linux API <linux-api@...r.kernel.org>,
Linux Containers <containers@...ts.linux-foundation.org>,
Akihiro Suda <suda.akihiro@....ntt.co.jp>,
Oleg Nesterov <oleg@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
Christian Brauner <christian.brauner@...ntu.com>,
Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace
On Sun, Oct 21, 2018 at 05:04:37PM +0100, Tycho Andersen wrote:
> On Wed, Oct 17, 2018 at 03:21:02PM -0700, Kees Cook wrote:
> > On Wed, Oct 17, 2018 at 1:29 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> > > On Thu, Sep 27, 2018 at 02:31:24PM -0700, Kees Cook wrote:
> > >> On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@...ho.ws> wrote:
> > >> > @@ -60,4 +62,29 @@ struct seccomp_data {
> > >> > __u64 args[6];
> > >> > };
> > >> >
> > >> > +struct seccomp_notif {
> > >> > + __u16 len;
> > >> > + __u64 id;
> > >> > + __u32 pid;
> > >> > + __u8 signaled;
> > >> > + struct seccomp_data data;
> > >> > +};
> > >> > +
> > >> > +struct seccomp_notif_resp {
> > >> > + __u16 len;
> > >> > + __u64 id;
> > >> > + __s32 error;
> > >> > + __s64 val;
> > >> > +};
> > >>
> > >> So, len has to come first, for versioning. However, since it's ahead
> > >> of a u64, this leaves a struct padding hole. pahole output:
> > >>
> > >> struct seccomp_notif {
> > >> __u16 len; /* 0 2 */
> > >>
> > >> /* XXX 6 bytes hole, try to pack */
> > >>
> > >> __u64 id; /* 8 8 */
> > >> __u32 pid; /* 16 4 */
> > >> __u8 signaled; /* 20 1 */
> > >>
> > >> /* XXX 3 bytes hole, try to pack */
> > >>
> > >> struct seccomp_data data; /* 24 64 */
> > >> /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
> > >>
> > >> /* size: 88, cachelines: 2, members: 5 */
> > >> /* sum members: 79, holes: 2, sum holes: 9 */
> > >> /* last cacheline: 24 bytes */
> > >> };
> > >> struct seccomp_notif_resp {
> > >> __u16 len; /* 0 2 */
> > >>
> > >> /* XXX 6 bytes hole, try to pack */
> > >>
> > >> __u64 id; /* 8 8 */
> > >> __s32 error; /* 16 4 */
> > >>
> > >> /* XXX 4 bytes hole, try to pack */
> > >>
> > >> __s64 val; /* 24 8 */
> > >>
> > >> /* size: 32, cachelines: 1, members: 4 */
> > >> /* sum members: 22, holes: 2, sum holes: 10 */
> > >> /* last cacheline: 32 bytes */
> > >> };
> > >>
> > >> How about making len u32, and moving pid and error above "id"? This
> > >> leaves a hole after signaled, so changing "len" won't be sufficient
> > >> for versioning here. Perhaps move it after data?
> > >
> > > Just to confirm my understanding; I've got these as:
> > >
> > > struct seccomp_notif {
> > > __u32 len; /* 0 4 */
> > > __u32 pid; /* 4 4 */
> > > __u64 id; /* 8 8 */
> > > __u8 signaled; /* 16 1 */
> > >
> > > /* XXX 7 bytes hole, try to pack */
> > >
> > > struct seccomp_data data; /* 24 64 */
> > > /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
> > >
> > > /* size: 88, cachelines: 2, members: 5 */
> > > /* sum members: 81, holes: 1, sum holes: 7 */
> > > /* last cacheline: 24 bytes */
> > > };
> > > struct seccomp_notif_resp {
> > > __u32 len; /* 0 4 */
> > > __s32 error; /* 4 4 */
> > > __u64 id; /* 8 8 */
> > > __s64 val; /* 16 8 */
> > >
> > > /* size: 24, cachelines: 1, members: 4 */
> > > /* last cacheline: 24 bytes */
> > > };
> > >
> > > in the next version. Since the structure has no padding at the end of
> > > it, I think the Right Thing will happen. Note that this is slightly
> > > different than what Kees suggested, if I add signaled after data, then
> > > I end up with:
> > >
> > > struct seccomp_notif {
> > > __u32 len; /* 0 4 */
> > > __u32 pid; /* 4 4 */
> > > __u64 id; /* 8 8 */
> > > struct seccomp_data data; /* 16 64 */
> > > /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
> > > __u8 signaled; /* 80 1 */
> > >
> > > /* size: 88, cachelines: 2, members: 5 */
> > > /* padding: 7 */
> > > /* last cacheline: 24 bytes */
> > > };
> > >
> > > which I think will have the versioning problem if the next member
> > > introduces is < 7 bytes.
> >
> > It'll be a problem in either place. What I was thinking was that
> > specific versioning is required instead of just length.
>
> Euh, so I implemented this, and it sucks :). It's ugly, and generally
> feels bad.
>
> What if instead we just get rid of versioning all together, and
> instead introduce a u32 flags? We could have one flag right now
> (SECCOMP_NOTIF_FLAG_SIGNALED), and use introduce others as we add more
> information to the response. Then we can add
> SECCOMP_NOTIF_FLAG_EXTRA_FOO, and add another SECCOMP_IOCTL_GET_FOO to
> grab the info?
>
> FWIW, it's not really clear to me that we'll ever add anything to the
> response since hopefully we'll land PUT_FD, so maybe this is all moot
> anyway.
I guess the only argument against a flag would be that you run out of
bits quickly if your interface grows (cf. mount, netlink etc.). But this
is likely not a concern here.
I actually think that the way vfs capabilities are done is pretty
nice. By accident or design they allow transparent translation between
old and new formats in-kernel. So would be cool if we can have the same
guarantee for this interface.
Christian
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